There are basically two types of plants that live on land: those with vascular tissues (xylem and phloem for carrying water and nutrients through the plant’s body) and those that do not. The first plants to conquer the land were nonvascular. These are the mosses, liverworts and hornworts, collectively known as the bryophytes. The development of vascular tissues in the ferns and their use in higher plants, the gymnosperms and angiosperms, saw the early pioneer species overshadowed in most ecosystems. But the bryophytes have continued to survive and the mosses in particular are still important components of many ecosystems. There are about 18000 species worldwide and they live just about everywhere, from the edges of Antarctica to the peat bogs in tundra and taiga ecosystems far to the north. They are found from deserts to rainforests and grow on a variety of substrates but because they have no true roots, most remain near sources of water.
The first plants were the algae and seaweeds and they were and are still restricted to aquatic environments. The liverworts were probably the first plants to leave the water behind and they show the closest relationships with green algae. The most diverse group of bryophytes though are the mosses. Although the bryophytes evolved from green plants and were the first to conquer land, they were probably not the direct ancestors of higher land plants: ferns, conifers and flowering plants,
Bryophytes are first of all green, having chloroplasts in their cells and producing their own food from sunshine. They are sexual beings, producing gametes in sexual organs and the zygotes develop in the female sex organ as embryos. They do not have true roots or stems. Instead bryophytes have filamentous rhizoids to hold them to their substrate but which do not transport nutrients or water to the rest of the plant as true roots do. There are no vascular tissues such as seen in higher plants which have xylem and phloem in their hard stems that allow them to grow much taller than the ground-hugging bryophytes. Bryophytes have no lignin, the complex carbohydrate that higher plants use to create hard upright structures such as stems, trunks and branches.
Liverworts and hornworts are neither numerous nor economically important but some mosses are. Sphagnum moss is estimated to cover one percent of the world’s land surface and has been important economically for thousands of years, because it forms peat bogs that can be harvested, dried and burned as a fuel.
The most interesting thing about bryophytes is their sex life. Each species comes in two forms: the gametophyte or sexual generation and the sporophyte or asexual generation. The plant we see is the gametophyte stage which has male and/or female parts. Water is needed to carry the male ciliated gametes to the female’s egg. The zygote grows into a diploid sporophyte which remains attached to its mother, which provides it nutrition as it has no leaves. It then produces spores and the life cycle continues.
References: http://bryophytes.plant.siu.edu/bryojustified.html http://www.hcs.ohio-state.edu/hcs300/liver2.htm